Ultraviolet Sterilization is a proven method of controlling waterborne planktonic algae and other harmful pathogens associated with aquatic recirculating systems, such as ponds, when and if it is used correctly. It, unlike other possible remedies, such as ozone or chemicals, has no residual life in the pond and is harmless to everything that does not get direct and close exposure to the radiation that they emit.
The UV lamp is the source of UV-C light and they can be either low pressure lamps or medium/ high pressure lamps. Low pressure lamps are generally considered the best for aquatic applications because most of their UV output is in the range of 254 nanometers, closest to what we need, and have a much longer life than medium/high pressure lamps. But all low pressure lamps are not created equal.
The intensity or strength of the UV light that is transmitted by the lamp is usually expressed as “microwatts per square centimeter of the lamps surface (Mw/cm2) and does vary tremendously. It is one of the first things to look at when comparing UV’s. A lower output lamp may be in the range of 20-25000 Mw/cm2 while a higher output lamp in the 75-100,000 Mw/cm2 range. So one 75000 MW/cm2 lamp can produce the same intensity of three 25000 Mw/cm2 lamps. Higher output lamps will cost more than the lower output lamps but only one is required, not three.
It is important to remember that the UV energy must hit the targeted organisms to be effective. The first hurdle is the bulb glass itself. It can be less expensive soft glass or higher quality hard quartz glass. The quartz sleeve and the transmission rate through the quartz sleeve can also vary a great deal, and again, that depends on the quality of quartz used. Higher quality generally means higher transmission rates, higher efficiency, but higher acquisition cost in both cases.
Dwell time or flow rate or exposure to UV light is also very important, maybe the most important. In order to be effective targeted organisms need to be exposed to a certain intensity of UV light for a certain amount of time. There is certainly a formula for this relationship but it is highly technical and is really not pertinent in this discussion. What’s important is that we recognize the direct relationship between intensity and dwell time or flow rates. If intensity is increased than flow rates can be increased, if intensity is decreased than flow rates must be decreased to obtain like results. So the lower intensity 25000 Mw/cm2 lamp could do the job but the flow rate would have to be less than the high output lamp.
The intensity of all UV bulbs declines with use, some as much as 40% in as little as 6 months. So a bulb may have a projected effective life 8,000 hours but we know that the intensity is declining. It is difficult to measure the actual intensity of UV energy hitting the water so observation is about the best we can rely on. It would be great if all manufacturers would give us Mw/cm2 of the bulbs they use when new and at the end of their effective life. But, in the meantime, if the water starts turning green it is time to change the bulb and/or clean it and the quartz sleeve. That may be once a year or more often or less often. Change it when it needs to be changed! My experience with UV’s indicates that the degradation accelerates with the age of the bulb so keep a replacement on hand. It is much easier to maintain clarity as opposed to cleaning
Electronic ballast are my choice in UV’s. They are much more efficient than magnetic ballast in converting input power to the proper lamp power. They can improve the overall lamp-ballast efficiency by as much as 15 to 20%.
The vessel itself can influence the efficiency of any UV. As discussed above in order to kill the targeted organism the UV light must hit them. Anything that reduces intensity, and exposure, by absorbing the light, adversely affects that ability. Whether it be poor quality glass, dirty glass or murky water or the vessel itself. Stainless steel, especially when the inside surface is polished, is highly reflective and therefore much less likely to absorb the UV light. Some feel that stainless steel can increase unit efficiency by 20% or more. It is also more expensive.
There are two other things that are worthwhile additions to any UV. They are an on/off switch and a pressure switch. The electronics in UV’s don’t respond well to quick on and off cycling. So when, for instance, you are back washing a bead filter the UV power cord should be unplugged. A pressure switch will shut the UV down as soon as water flow stops or slows down to an unacceptable level. If the unit continues to operate in either one of these scenarios the heat generated will quickly destroy the unit.